The reason I jumped on this thread with such interest is that I have several projects going on currently that have a similar concept. Here is my thread for a pair of 200W monoblocks using parallel output iron:

I already have all the parts on hand, so for me it's just a challenge. 125 WPC with Pete's 18 WPC board was too easy. I ran the board at 250 WPC for several days with no incidents, but I think pushing 250 WPC through Pete's board was a bit too far. There are some traces that are just too close for comfort.

There have been requests for BIG power amps, but the reality usually sets in when the $$$$ and the risk of failure starts tallying up. Search this forum and you will find very few tube amps over 200 WPC have been built. There are reasons for this. Many people underestimate the cost, risk, and probability of failure involved with building a big tube amp. All go up exponentially with the power level. Too many people start building something beyond their ability, and never finish, or worse blow up some very expensive parts, then give up.

The traditional method for big power is big voltage and transmitter tubes. This means high impedance OPT's which are hard to make, and expensive. It is also possible to run multiple lower powered tubes in parallel, reducing the impedance and the voltage level, but the tube costs go up.

Still to make a 500 WPC amp, we are dealing with a B+ of 600 volts, and currents over 1 AMP. Yes, this can kill you, and then cook your body before the power fuse blows!

Do I have the experience to design and make this amp? YES. Do I have all the parts? YES? Do I need a big amp like this? NO. Do I want to make it just because I can? Maybe. Would a BIG A$$ guitar amp be cool???? Is the Brownout 1000 cool? YOU BET!

If you haven't seen it before, Yes I went down the transmitting tube route before too! Totally not practical.

To produce that kind of power would not take that many 6C33s if you ran OTL. Also, the 'fixed output impedance' thing is not a concern.

We built a 500-watt OTL using 6AS7s: Atma-Sphere music systems, inc. That took only 42 tubes per channel, you could make 900 watts with 90 6AS7Gs. The 6AS7G is still being made.

Agreed. OTL seems to be the way to go for high power. 2 6C33C's will do 25W, and roughly speaking, the maximum power scales as the square of the number of tubes (because the maximum power is limited by the current the tubes can pass). So 4 6C33C's would be approaching 100W, and 8 could do roughly 400W or so. (That's probably a bit of an overestimate, but not too far off the mark.)

Alan Kimmel's circlotron, using EL509 sweep tubes, is apparently 225W using 8 tubes. High power is pretty easy with OTL, and relatively cheap because of the savings in OPT's.

Like I said, for 500 watts you can do it with forty 6AS7Gs/channel. That is still a big amp- the version we did had outboard power supplies and required automatic variacs controlling bucking transformers so that when the amps dimmed the AC power line, they could still make full power.

A class AB2 833A2 could esaily surpass 500W. It can be done but // tubes is much easier, however more prone to decreased reliability. I developed a 150SE triode amplifier that never failed from the very first start-up years ago.